Question

I'm having some troubles with these. Thanks in advance.

If a>0 find the minimum value. If a<0 find the maximum value.

1. y=-x²+2x+5
2. y=-2x²-3x+4

These are functions for parabolas, so
f(x)=ax²+bx+c when a does not equal 0

In both your examples, the "a", which is the coefficient of the x^2 term, is negative.

1. dy/dx = -2x + 2, and that equals zero at a maximum or minimum. So the maximum would occur at x=1.
d2y/dx^2 = -2. Since that is always negative, x=1 is a point of maximum value.

Do #2 similarly.

Thank-you, but I really don't understand what you mean by dy/dx. I'm feeling terribly stupid, but nothing like that is in my book.

Find Minimum or maximum value of f(x)=2x^2 + 3x -9